Determining fracture mode transition behavior of solid materials using miniature specimens

ABSTRACT

A method of determining fracture mode transition behavior (FMTB) of solid materials by using stress field modified miniature specimens. The method is an improvement in the method of determining mechanical behavior information from specimens only so large as to have at least a volume or smallest dimension sufficient to satisfy continuum behavior in all directions. FMTB of the material is determined from the measurements taken during the loading of the specimen resulting in the formation of cracks and/or the further propagation of cracks in the miniature specimen and/or fracture. The specimens are provided with grooves that induce additinal stress field modifying stress components in the specimens during the test. These additional stress components result in a desired stress state in the specimen which could not be achieved otherwise. The methods are useful in determining FMTB for the material, when the specimen thickness is smaller than previously thought necessary for valid FMTB determinations.

FIELD OF INVENTION

This invention relates to methods and apparatus for determining themechanical behavior of solid materials and is especially useful fordetermining and measuring the mechanical materials when loaded orstressed for the purpose of establishing the design, use, safelife, andpost-service criteria of the material. Although the term miniature isrelative, as are all size descriptive terms, it is a faircharacterization to define the field of this invention as thedetermination of the mechanical behavior of materials from miniaturespecimens, i.e., specimens noticeably smaller than prior conventionalspecimens in the materials testing field. More particularly, thisinvention relates to the testing of material specimens of thickness andsize less than thought necessary for valid determinations of fracturemode transition behavior (FMTB), such as ductile-brittle transitiontemperature (DBTT) of solid materials, including but not limited toferritic steels.

BACKGROUND OF THE INVENTION

Determination of the mechanical behavior physical properties ofmaterials is necessary so that materials may be selected for use,evaluated when in use, and evaluated after use. From thesedeterminations, decisions are made as to which materials to use, theconditions under which they can be used, and whether such materials inuse can be continued to be used with safety. These types ofdeterminations are particularly useful for determining the effects ofenvironmental loading such as nuclear radiation on the mechanicalproperties of in-service materials. This invention is fully applicableto the determination of mechanical behavior of such materials but isalso applicable to materials not subjected to radiation and the validityof the invention was demonstrated for materials not subjected toradiation.

The prior art includes U.S. Pat. No. 4,567,774 having the inventor,Michael P. Manahan, common with this application, and assigned to thesame assignee. This earlier patent, hereinafter referred to as the"prior patent", includes the basic concepts upon which this invention isbased. The disclosure of the prior patent is included herein byreference and the portions of that disclosure not specifically neededfor the disclosure of this improvement invention are not includedherein. However, reference to the prior patent may be helpful to theunderstanding hereof.

In the past, the most common procedure has been to determine themechanical behavior of materials by testing large samples that areeither created more or less simultaneously or side by side with theproduct that is intended to be used or are cut from the same batch ofmaterial. In the determination of the mechanical behavior of solidmaterials and particularly metals, the practice is to make tensile,fatigue, creep, stress relaxation, ductile/brittle transition behavior,fracture toughness, etc. specimens; and these are then subjected toloads while measurements are taken of the force, time, displacement,temperature, energy, velocity, crack length, etc. of the specimen.Information on stress and strain which can be thought of as normalizedload and deflection respectively as well as other useful parameters arethen obtained by simple mathematical operations. For example, in auniaxial tensile test the stress is determined by dividing the measuredload by the specimen cross sectional area.

While this may be satisfactory in most instances, there are othercircumstances such as the post-irradiation testing of materials used innuclear reactors where samples may be unavailable in sufficient size andquantity to carry out these destructive tests during the life of thematerials in use. In general, neutron irradiation space for materialsinvestigations is limited and costly. It is, therefore, desirable to usespecimens of minimum volume. Since neutron irradiation costs scale withspecimen volume, miniaturized mechanical behavior testing can providesignificant savings in irradiation testing costs for nuclear materialsinvestigations. In addition, it is possible to provide mechanicalbehavior information which is not ordinarily obtainable due to spacelimitations in irradiation experiments and thus expedite alloydevelopment investigations. Of course, miniature specimen testing isapplicable to materials investigations for other nuclear technologies aswell as non-nuclear technologies requiring mechanical behaviorcharacterizations from a small volume of material. One such non-nuclearapplication is cutting small pieces of material from in-servicecomponents and using miniature specimens to measure the currentmechanical behavior state. These data can then be used to estimate theremaining life of the in-service component.

Alternatively, a miniaturized bend test can be employed wherein thefracture mode transition behavior for the material is determined bysuspending the specimen between two spaced apart points whilesimultaneously bringing down a substantially centrally-positioned punchonto the notched and/or precracked specimen to deflect it and byproviding other modifications to the specimen to achieve reasonably flatfracture surface and sufficient constraint so that fracture modetransitional behavior can be measured. This type of test can becharacterized as a three point bend test. The present invention wasconceived as a solution to the problem of determining the fracture modetransition behavior from miniature specimens which are thinner andsmaller in volume than those conventionally employed in the art. Moreparticularly, this invention relates to the testing of materialspecimens of thickness less than thought necessary for validdeterminations of ductile-brittle transition temperatures (DBTT)particularly, and less than the minimum size as taught by ASTM A370-77,E23-86, and E812-81.

There are four principal conceptual innovative aspects to theminiaturized fracture mode transition behavior (FMTB) testing method ofthis invention. The first is the use of specimens that are significantlythinner and smaller than those currently in use. The second is the useof an experimental stress field modifying technique so that useful datacan be obtained using small specimens. A particular manifestation isapplication of a transverse load in the thickness direction and/ormaking side grooves and spacing them such that a nearly constant throughthickness stress field of sufficient magnitude so as to yield usefuldata is achieved. The third is the use of conceived and verifiedanalysis techniques to produce results that have significant, adequate,and useful correlation with the results that are obtained by specifiedASTM testing methods such as ASTM E23-86. The fourth is the use of thefinite element method to calculate the direction and amount ofadditional load and/or side grooving to be applied to achieve thedesired stress state in the specimen. In a particular manifestation, thefinite element code is used to determine the amount of side grooving toachieve sufficient transverse tensile stress such that a reasonably flatfracture surface can be obtained.

This invention improves upon the method of U.S. Pat. No. 4,567,774 byteaching the modifying of the stress field during the testing with theminiature specimen. This modifying of the stress field can be donemechanically, or by using a force field such as a magnetic field inorder to produce stresses in preferred orientations in the material, orby causing a change in the stress field conditions by the means of theremoval of material on the sides of the specimen. Since plane strainneed not be achieved in fracture mode transition behavior testing, sidenotching of the specimen is the preferred approach since this isexperimentally less complicated. The material is removed in the form ofa groove or crack on each side of specimen. In essence, the stress fieldmodification replaces the need for material thickness.

In one particularly usefully test configuration, the miniature specimenis loaded in a three point bend test and parameters such as load,deflection, temperature, and time are measured. The data are thenanalyzed using either fracture appearance as the correlation parameter,or percent post-maximum energy as the new parameter. The data analysisprocess using this new parameter is necessary in order to obtainconventional full size energy vs temperature Charpy data as described inASTM E23 using the miniature specimen data.

In essence, the stress field modification replaces the need for materialthickness. The stress field is modified by providing grooves on twosides opposite to the notch to provide overlapping stress fields thatinclude transverse stress components.

Current test procedures require a minimum specimen thickness whichcannot be satisfied in many cases. This serves to preclude use ofminiature specimens. The advantage of the present invention is thatspecimens which are much smaller than those currently in use can beaccurately tested. This enables testing of materials removed fromin-service components in cases where it is not possible to remove enoughmaterial to meet current ASTM test requirements. The invention can alsobe used to provide additional nuclear pressure vessel surveillance databy cutting miniature specimens from the broken halves of full sizecharpy specimens. Another advantage of the method of the invention isthat the method allows the modifying of the stress field such that mixedmode fraction in the transition region can be avoided or conservativelyaccounted for. The invention enables restricting fracture in the stressregion to fixed mode fracture.

Another advantage of the present invention is that in-service stressfields can be simulated in the laboratory thereby providing data whichis more representative of component performance.

This invention enables the determination of the DBTT of materials fromminiature specimens; i.e. specimens noticeably smaller than priorconventional specimens in the materials testing field. Moreparticularly,this invention relates to the testing of material specimensof a size, approximately one twentieth by volume more or less thanthought necessary for valid determination of ductile-brittle transitiontemperature (DBTT) of solid material determined using Charpy specimens.

SUMMARY OF THE INVENTION

In summary, this invention is a process of determining the mechanicalbehavior of solid materials, comprising: (a) providing a specimen of thematerial having a volume and smallest dimension sufficient to establishcontinuum behavior in all directions, and with a volume not more than10₇ times said sufficient volume (b) providing a specimen of thematerial having a notch on one side to provide a stress concentration toinitiate cracking; (c) modifying the stress field of the specimen byproviding grooves on the two sides opposite to the notch to provideoverlapping stress fields including transverse stress components whichare approximately equal throughout the thickness, resulting inmeasurable fracture transition mode behavior; (d) modifying the stressfield of the specimen mechanically by providing a force in thetransverse direction or by using a force field such as a magnetic fieldin order to produce stresses in preferred orientations in the material;(e) deforming the specimen by applying a load on the specimen in adirection different than the orientation of the modified stress fieldload or stress field component from side grooves; (f) measuring at leastone key variable in step e; and (g) determining the behavior of thematerial from the measurements taken according to the principles of thefinite element method and/or the principles of linear or nonlinearmaterial mechanics or both.

This invention is directed to solving the specimen size effects problemin FMTB testing. The invention enables the use of specimens which aremuch smaller than those currently in use. As specimen thickness isreduced, the transverse stress component decreases and the stress fieldnear the crack tip changes from plane strain (tri-axial state of stress)to plane stress (predominantly bi-axial state of stress). Eventually athickness and size is reached where the specimen plastic deformation isexcessive and a reasonably flat fracture surface cannot be achieved andtherefore the data can not be analyzed in a meaningful manner. Also,FMTB can not be measured.

Therefore, the invention uses side-grooves to increase the transversestress, provide constraint, and enable the measurement of FMTB atspecimen sizes where it would not be possible otherwise. In the priorfracture toughness testing art, side grooving is employed with largespecimens for two basic reasons. The first is to keep a crackpropagating in a desired plane and the second is to flatten the throughthickness stress field in thick fracture toughness specimens so thatrelatively flat crack fronts are achieved. In this invention, sidegrooves are used for the first time in FMTB testing for the purpose ofinducing a fairly uniform transverse stress field by reducing thespecimen thickness or diameter to an optimum level so the stress fieldfrom the two notches overlap. This approach enable determination of FMTBin specimens which are thinner than required to obtain reasonably flatfracture surfaces which can be accurately analyzed.

Further, this invention provides a method of data analysis which enablesaccurate correlation between miniature FMTB specimens and full size ASTME23 specimens such as the Charpy specimen. The new parameter needed isthe percent post-maximum energy. The fracture surfaces are studied todetermining the proper post-maximum energy index to be used for analysisof the miniature FMTB data to correlate with conventional Charpy data.

A particular object of this invention is to provide a method ofdetermining the DBTT of solid materials from specimens with onlysufficient volume and smallest dimension to satisfy continuum behaviorin all directions. It is a feature of this invention to provide a methodof determining the DBTT of solid materials accurately by bending ortensile loading miniature specimens. Still a further feature is todetermine the DBTT accurately by the finite element method, particularlyto which a modified stress field has been applied along with theminiature bend test.

Another feature is to determine the mechanical behavior by the processesof continuum material mechanics carried out by a code which is appliedaccording to a predetermined algorithm which has been determined to bestatistically accurate.

An overall object of the invention is to provide the capability ofdetermining mechanical behavior of material through a process usingspecimen sizes so small that they may be trepanned from existingstructures without significantly altering the overall characteristics ofthe structures.

The foregoing and other advantages of the invention will become apparentfrom the following disclosure in which a preferred embodiment of theinvention is described in detail and illustrated in the accompanyingdrawings. It is contemplated that variations in structural features andarrangement of parts may appear to the person skilled in the art,without departing from the scope or sacrificing any of the advantages ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic elevational view of one of the fixtures andspecimens employed in the standard ASTM FMTB impact (Charpy) test usedto determine ductile-brittle transition temperature (DBTT).

FIG. 2 is a schematic enlarged perspective view of the hammer, anvil,and specimen support portion of the apparatus shown in FIG. 1.

FIG. 3 is an elevational view of one of the fracture mode transitionbehavior specimens (Charpy V notch specimen) specified for use in ASTME23-82, showing its position on the anvil.

FIG. 4 is a plan view of the specimen shown in FIG. 3.

FIG. 5 is an elevation view of the apparatus used to test specimens ofthe configuration shown in FIGS. 3 and 4; as well as the specimen shownin FIGS. 6 and 7 by the method of this invention.

FIG. 6 is an elevational view of a specimen used in the test methods ofthis invention.

FIG. 7 is a plan view of the specimen shown in FIG. 6.

FIG. 8 is an elevational view of the standard ASTM anvil used in thetest apparatus of FIGS. 1 and 2.

FIG. 9 is a plan view of the anvil shown in FIG. 8.

FIG. 10 is an elevational view of the anvil block used for bend testingminiature specimens in accordance with the methods of this invention.

FIG. 11 is a plan view of the anvil shown in FIG. 10.

FIG. 12 is an enlarged view of the supporting specimen ledge surface inthe apparatus shown in FIG. 10.

FIG. 13 is an elevational edge view of a punch used in the standard ASTMtest for specimens in methods to determine DBTT.

FIG. 14 is an elevational view of the edge of the punch used in themethod of this invention to determine DBTT of material by the use ofminiature specimens.

FIG. 15 is a graph of the normalized total energy vs percent shear asfracture transition criteria for Charpy specimens showing threematerials that were tested.

FIG. 16 is a graph of the normalized total energy vs percent shear asfracture transition criteria showing curves for three materials thatwere tested with miniature specimens.

FIG. 17 is a graph of a non-normalized total energy vs percent shear asfracture transition criteria showing curves for three materials thatwere tested with ASTM standard Charpy specimens.

FIG. 18 is a graph of the non-normalized total energy vs percent shearas fracture transition criteria showing curves for three materials thatwere tested with miniature specimens.

FIG. 19 is a graph of the pre-maximum and post-maximum load energies vstemperature for ASTM standard specimens.

FIG. 20 is a graph of the pre-maximum and post-maximum load energies vstemperature for miniature FMTB specimens.

FIG. 21 is a graph showing correlation of percent of post-maximum loadenergy as a fracture transition criterion with percent shear for bothASTM standard and miniature FMTB specimens.

FIG. 22 is a graph showing material comparisons of transitiontemperature using the 15% post-maximum load energy index having bothASTM standard and miniature FMTB specimens plotted as temperature vspercent post-maximum energy for one of the test materials.

FIG. 23 is a graph showing material comparisons of transitiontemperature using the 15% post-maximum load energy index having bothASTM standard and miniature FMTB specimens plotted as temperature vspercent post-maximum energy for another of the test materials.

FIG. 24 is a graph showing material comparisons of transitiontemperature using the 15% post-maximum load energy index having bothASTM standard and miniature FMTB specimens plotted as temperature vspercent post-maximum energy for still another test material.

FIG. 25 is a graph showing transition temperature using the four percentshear fracture index showing both standard ASTM specimens and miniatureFMTB specimens and plotted as temperature vs shear fracture appearancefor the same material as FIGS. 20 and 22.

FIG. 26 is a graph showing transition temperature using the four percentshear fracture index showing both standard ASTM specimens and miniatureFMTB specimens and plotted as temperature vs shear fracture appearancefor the same material as FIG. 23.

FIG. 27 is a graph showing transition temperature using the four percentshear fracture index showing both standard ASTM specimens and miniatureFMTB specimens and plotted as temperature vs shear fracture appearancefor the same material shown in FIG. 24.

FIG. 28 is a graph showing a comparison of upper self energies for eachof the test materials and each of the types of specimens plotted as testtemperature vs total energy.

FIG. 29 is a schematic representation of the transverse tensile stressfield component created at the notch when a specimen is loaded in themanner required for FMTB testing.

FIG. 30 is a schematic representation of the transverse tensile stressfield component across the width of an ASTM standard size fracturetoughness specimen when a specimen is loaded.

FIG. 31 is a schematic representation of the transverse tensile stressfield component showing the through thickness stress field when agrooved miniature specimen is loaded in the manner required for FMTBtesting.

FIG. 32 is a side view of another embodiment of the specimen useful inthe practice of this invention.

FIG. 33 is a cross sectional view of the embodiment shown in FIG. 32.

FIG. 34 is an end view of a specimen having a trapezoidal cross sectionfor use in another embodiment of this invention.

DETAILED DESCRIPTION OF THE INVENTION AND BEST MODE FOR CARRYING OUT THEPREFERRED EMBODIMENT

Referring to FIGS. 1 and 2, prior art ASTM methods and apparatus areshown for determining the FMTB of solid material in which the specimenis placed in an anvil 126 and struck by a hammer 127. The hammer 127 issuspended on an arm 130 that rotates on a trunion axis 129. The anvil126 is provided with a receptacle 131 to receive the specimen 125positioned against ledge faces 133.

As most clearly seen in FIGS. 3 and 4 the specimen 125 is provided witha notch 135 on one elongated side 130. In the ASTM specification E23-82,dimensions shown in FIGS. 3 and 4 are specified. The shape is alsospecified and comprises two pairs of elongated parallel sides includinga bottom 137 and a top 138 with sides 139 and 140. Square ends 141 and142 are parallel to each other. Punch 145 is positioned intermediate theends 141 and 142 on the top side 138 and has a configuration mostclearly shown in FIG. 13.

Referring to FIG. 5, a different apparatus for determining FMTB 150includes a fixed platen 151 and a movable platen 152 with the specimen125 mounted between. The specimen is mounted on an anvil support 154that is supported on platen 151 by an adjustable threaded member 155. Apunch 145 is mounted in an adaptor 156 that is connected to a load cell158 with a support 160 that is fastened to the upper movable crosshead152. The apparatus 150 is used to provide a bend test to fracture whichis used as another method embodiment in determining DBTT. In operationthe movable platen 152 is lowered bending the specimen 125 andfracturing the bottom side 137 at the notch 135.

As seen in FIGS. 8 and 9 the dimensions and proportions andconfiguration of the anvil 154 provide ledges 162 on opposite sides tosupport the specimen 125 near the ends 141, 142 (see FIG. 3).

In the preferred embodiment, miniaturized FMTB three point bendspecimens are tested to obtain ductile-brittle fracture transitioninformation. Standard-size Charpy V notch (CVN) bars were also tested toprovide a comparison with the miniature specimens. Table III.1 comparesthe miniature specimen dimensions with those of the conventional ASTMCharpy specimen. The Specimens are shown in FIGS. 3, 4, 6, and 7.

The smaller thickness of the miniature specimen results in a lowertransverse stress field than that present in a standard CVN specimen. Itis important to consider the effects of several variables such as notchacuity, strain-rate, and the size effect on the experimental results tobe able to interpret them. While it is relatively easy to takestrain-rate effects into account by means of a temperature translationin the energy temperature curves, the treatment of notch acuity and sizeeffects is less straightforward. As stated in the prior art, there is achange in the fracture initiation micromechanism from microvoidcoalescence to intergranular fracture as the notch root radius increasesbeyond a certain critical value. It is therefore important to becertain, in comparing the results of standard and miniature tests, thatthe basic mechanisms of deformation and fracture are comparable.

                  TABLE III.1                                                     ______________________________________                                        COMPARISON FF CONVENTION ASTM CHARPY                                          SPECIMEN WITH MINIATURE SPECIMEN [MANA85]                                                 ASTM                                                                          Charpy Specim   Mini     Specim                                               (mm)   (in)     (mm)     (in)                                     ______________________________________                                        Thickness (B) 10.01    (0.394)  4.83   (0.190)                                (Crack Plane)                                                                 Depth (H) direction                                                                         10.01    (0.394)  4.83   (0.190)                                of crack propagation                                                          Length (L)    54.99    (2.165)  12.70  (0.500)                                Reduced Side  n/a      (n/a)    3.86   (0.152)                                Thickness B                                                                   Notch Depth (a) 25                                                                          2.01     (0.079)  0.97   (0.038)                                Notch-Root Radius(r)                                                                        0.25     (0.010)  0.25   (0.010)                                ______________________________________                                    

The steel tested is a reactor-grade ASTM A508 steel, designated TSE-6,.For the purposes of simulating irradiation-induced embrittlement,different heat treatments were applied to the steel, to produce threedifferent degrees of temper-embrittlement.

The miniature FMTB specimens were conceived on the basis of:microstructure and desired stress state. The lower bound on the minimumspecimen dimension is dictated by the size of the largestmicrostructural inhomogeneity. This steel was found to have carbon-richregions about 0.25 mm (0.010 in) and 0.5 mm (0.020 in) apart. For thespecimens to be representative of the general behavior of the steel, thecrack plane must be, at a minimum, 3 to 5 mm (0.12 to 0.20 in) or 5 to10 times the characteristic inhomogeneity dimension.

The miniature FMTB tests can be performed at any experimentallyachievable loading rate by varying the punch velocity. The miniatureFMTB tests reported herein were of the notched-bar three-point bend typeand were performed at static (slow-bend) loading rates. Standard Charpyv-notch specimens were tested dynamically and in slow bend on an anvildesigned to conform to ASTM E23, which is the standard for notched-barimpact testing (See FIGS. 8 and 9). Procedures relating to testtemperatures, alignment accuracy, and machining tolerances werefollowed. Table III.2 shows the relevant anvil support and punchdimensions, and the anvil and support are shown in FIG. 10, 11, 12, 13,and 14.

                  TABLE III.2                                                     ______________________________________                                        RELEVANT ANVIL SUPPORT AND                                                    PUNCH DIMENSIONS                                                                       Conventional                                                                  Charpy Test                                                                   [ASTM81]     Miniature                                                        (mm)  (in)       Specimen Test                                       ______________________________________                                        Punch      8.00    (0.315)    0.64  (0.025)                                   Radius                                                                        Punch Tip  3.99    (0.157)    1.27  (0.050)                                   Width                                                                         Anvil      1.00    (0.039)    0.13  (0.005)                                   Radius                                                                        Anvil      40.00   (1.575)    11.68 (0.460)                                   Spacing                                                                       ______________________________________                                         Note: Both Anvils were fabricated from Viscount 44 steel. The punch was       made of Vega steel, Rockwell 60 hardness.?                               

As the specimen thickness decreases, the stress field close to the notchbecomes less triaxial (plane-strain) and more biaxial (plane-stress). Itis possible to modify specimen design to increase constraint. Theapproach used was to introduce side-grooves into the miniaturespecimens. The side-grooves introduce a transverse tensile stress fieldwhich acts to increase the through-thickness stress near the notch, thushelping to increase the triaxiality in the vicinity of the notch. Thespecimen thickness was reduced such that the side groove stress fieldsoverlap and results in a significant through thickness stress field.While it is not possible to achieve plane strain conditions at most testtemperatures, sufficient constraint is added so that fracture transitionbehavior can be measured. The through thickness component of stress fora notch is shown schematically in FIG. 29. The decrease of the throughthickness stress field due to plastic constraint is shown schematicallyin FIGS. 30 and 31. The concept of bringing the side grooves into nearproximity to modify the transverse stress in a miniature FMTB specimenis shown in FIG. 31.

Stress-field modification characterization may be accomplished usinganalytical methods such as finite-element analysis. Alternatively theeffect of side-grooving and miniaturization on the stress state (andtherefore on the fracture behavior) of the miniature specimens can beinvestigated by experimentation. Miniature specimens with differentside-groove dimensions (depth and root radii) were tested at roomtemperature and using the same material (60R-93). Table III.4 gives alist of the various side-groove dimensions used in testing.

                  TABLE III.4                                                     ______________________________________                                        SIDE-GROOVING STUDY                                                           ______________________________________                                        Miniature Specimen Identifying Numbers                                        8a    3        4       5      6a    7      2                                  Pre-Maximum Load Energy (kJ/m.sup.2)                                          971.18                                                                              918.11   843.26  804.41 767.46                                                                              735.24 668.92                             Side-Groove Depth (mm)                                                        0.0   0.24     0.24    0.24   0.49  0.49   0.49                               Side-Groove Root Radius (mm)                                                  n/a   0.25     0.13    0.04   0.25  0.25   0.04                               Notch Root Radius (mm)                                                        0.25  0.25     0.25    0.25   0.13  0.04   0.25                               ______________________________________                                    

All of the side-grooving tests were performed on the miniature 60R-93specimens at room temperature (68° F.). None of the tests resulted inany brittle fracture. However, it was observed that pre-maximum loadenergy, obtained prior to major specimen deformation, could yieldinformation on crack-initiation processes. The pre-maximum load energiesshowed a regular increase as the depth of the side-grooves decreased.The specimen without side-grooves (8a) exhibited the highest pre-maximumload energy. Among specimens with the same sidegroove depth, there is aweaker correlation with side-groove root radius. Thus in specimens 3, 4and 5, the pre-maximum load energy decreases as the side groove rootradius decreases. Among the remaining specimens (6a, 7, 2), all withdeep side grooves (0.49 mm), the sensitivity to side-groove root radiusor notch root radius is less apparent. Table III.4 gives the variousnotch and side-groove dimensions as a function of decreasing pre-maximumload energy.

An important finding was that the specimens without side-groovesdeveloped crack planes which did not stay parallel to the plane normalto the specimen length. None of the specimens with side-groovesexhibited this behavior. A conclusion to be drawn is that side-groovesare essential to testing miniature FMTB specimens, the constraintintroduced by side grooving is sufficient to measure transitionbehavior.

The main findings of the side-grooving study are:

(1) Side-grooves are necessary to keep the crack propagation directionapproximately in the plane normal to the specimen length. The specimenswithout side-grooves showed crack planes which did not follow thestraight line from the notch to the punch.

(2) Crack-initiation energy is dependent on specimen geometry and theside-groove dimensions. The pre-maximum load energy was found todecrease with increasing side-groove depth and decreasing side-grooveroot radius.

(3) Since side-grooving changes the stress field at the ends of thenotch, there could be an effect on the crack front, and hence onfracture appearance.

As described earlier, three heat treatments were applied to the ASTMA508 steel made, each resulting in a different temper-embrittlement. Thematerials were designated as:

60R-93 for Heat Treatment 6 [Tempered at 613° C. (1135° F.) for 4 hours,furnace-cooled]

5A-91 for Heat Treatment 5A [Tempered at 679° C. (1254° F.) for 4 hours,furnace-cooled]

60RR-5 for Heat Treatment 6R [Tempered at 705° C. (1301° F) for 4 hours,furnace-cooled]

These are in order of increasing toughness. The 60R-93 material showedthe most brittle behavior while the 60RR-5 exhibited the least.

Several parameters of interest can be extracted from bend testload-deflection curves, such as:

(a) Total absorbed energy to fracture

(b) Energies associated with crack initiation and propagation

(c) Yield load

(d) Maximum load

(e) Fracture load

(f) Deflection at brittle fracture

Energies are obtained from the load/deflection curves by integration(i.e., by measuring the areas under the curves). A planimeter can beused for this purpose. With static testing the punch displacementvelocity is constant, and it is not necessary to make corrections forthe slowing-down of the striker as in impact testing.

Fracture-appearance measurements can be made from photographs of thebroken surfaces of the specimens and/or by observation of the actualfracture surfaces under a stereomicroscope. Percent areas of fibrous(ductile) and cleavage (brittle) fracture are measured from magnifiedphotographs by means of a planimeter. Fibrous or ductile fracture areasare identified by their dull, "torn" appearance; while brittle orcleavage-type fracture areas have a shiny, "faceted" appearance. Otherfracture appearance features noted are shear-lips, which have a fibrousappearance.

An important aspect of the fracture appearance is the shape of the crackfront. The conventional Charpy specimens exhibit a convex crack frontwhen viewed with the notch closest to the observer. This indicates thatthe crack initiates near the center of the notch where the stresses arehighest. The miniature specimens show a concave crack front, suggestingthat crack initiation occurs near the side grooves. The presence of theside-grooves in the miniature specimens results in an increase in thestress level near the side grooves, and the stress may be high enough toinitiate the crack in this region of the specimen. The prior art statesthat these differences in crack front shape indicate that the appearanceof the full-width crack at the notch root does not necessarily coincidewith maximum load. This can be assumed to be the case or the crackformation can be measured by a technique such as electricpotential (EP).The assumption is particularly valid near the lower shelf, which is oneregion of interest for conventional Charpy testing.

Small notched specimens exhibit greater plasticity due to lack ofmaterial (thickness) constraint. The miniature specimen geometry ismodified by machining side grooves into the specimens and adjusting thespecimen thickness with the objective of increasing the transversestress component to approach a higher degree of triaxiality of thestress field near the notch. This technique induces cleavage, mixedmode, and ductile fracture in the miniature specimens at temperaturesreadily achievable in the laboratory, and produces relatively flatfracture surfaces. A temperature correction is applied to obtainconventional FMTB and, in particular, Charpy data.

It is recognized that the amount of transverse constraint achieved inthe miniature specimens by the side grooving technique is arbitrary.Plane-strain conditions are not in general achieved except perhaps atvery low temperatures, and the stress field in the vicinity of the notchis different in the miniature and conventional specimens. However, thedegree of constraint achieved is enough to obtain a ductile-brittletransition.

The miniature test results are used to obtain an estimate of the Charpy30 ft-lb dynamic transition temperature. The absorbed energy can benormalized by the crack plane area and used as the Charpy parameter. The41 Joule (30 ft-lb) Charpy index used to define the transitiontemperature is normalized by the same factor, i.e., by the crack planearea. This results in a normalized Charpy index of 512 kJ/m². To allow adirect comparison between the heats, all the test temperatures for theslow-bend tests are adjusted by subtracting the appropriate conventionalCharpy impact transition temperature. These impact transitiontemperatures are given in Table IV.1. Using the 512 kJ/m² level as anindex for both specimen sizes enables correction factors to beestablished to account for specimen size and loading rate for both theminiature and the standard CVN data.

                  TABLE IV.1                                                      ______________________________________                                        CONVENTIONAL CHARPY IMPACT                                                    TRANSITION TEMPERATURES                                                                     Temperature                                                     Material        (C.)   (F.)                                                   ______________________________________                                        60R-93          40     (104)                                                  5A-91           -7     (19)                                                   60RR-5          -29    (-20)                                                  ______________________________________                                    

The superposition of data that results from this procedure establishesthe rate-effect correction factor in the 41-J transition temperature at45.3 C. Thus, there is a downward shift of 45.3 C in the 41-J DBTT ingoing from dynamic to static test conditions for conventional CVNs.Following the same procedure for the miniature specimens results in adownward shift of 122 C in going from dynamically tested conventionalCVNs to statically tested miniature CVNs. Since 45.3 C of this shift isdue to the rate effect, the remaining 76.7 C is attributed to the sizeeffect in going from conventional to miniature specimens in slow-bend.

While the energy normalization approach yields reasonable data, it isnot known to what extent the fracture mechanism exhibited by theminiature specimens are always representative of the standard CVNs. Itis essential that the fracture mechanisms or modes be similar in bothsizes of specimen for the 512 kJ/m² index or any other index. Of generalconcern is the increased ductility exhibited by miniature specimens, andthe extent to which this is remedied by the presence of the sidegrooves. Investigation of the normalized energy parameter and the 512kJ/m² index revealed that they are not, in general, correct forcorrelating miniature FMTB and conventional Charpy data. This isdescribed below.

It is necessary to determine whether the fracture mechanism is the samefor the 512 kJ/m² index in both standard and miniature specimens. Onlyin this case would the 76.7 C size effect correction have a physicalmeaning.

Therefore fracture appearance was used as a direct measure of thevalidity of parameters to characterize the ductile-brittle transition.It was recognized that this parameter is valid regardless of specimensize, and that a quantitative measure of the ductile-brittle transitioncould be obtained if accurate measurement techniques are used. The ASTMStandard E-23 for Charpy impact testing gives four optional techniquesfor measuring fracture appearance: (1) a table can be used to convertlinear measurements of the fracture surface areas to percent shear; (2)the appearance of the fracture surface may be compared with a pictorialchart of fracture appearance; (3) the fracture surface may be magnifiedand compared to a precalibrated overlay chart; or (4) magnifiedphotographs may be measured by means of a planimeter.

The first three of these methods do not give very accurate estimates ofthe percent shear. Magnified photographs of the fracture surfaces can bemeasured by means of a planimeter, in conjunction with observations ofthe actual specimens under a stereoscope to obtain good accuracy.

Fracture appearance is not generally used as a measure of theductile-brittle transition due to the subjective nature of the usualmethods of measurement, and to the fact that it requires considerableeffort to obtain accurate data. Several industries, such as the nuclearindustry, require by codes and standards the measurement of Charpyimpact energy vs. temperature data.

The normalized energy data and the fracture appearance data were fit andthe results are shown in FIGS. 15 and 16. It is possible to plot asingle curve with reasonable accuracy for the standard or the miniaturespecimens for all 3 materials. However, it is obvious that the curvesfor the two specimen types are not coincident for both the energy andnormalized energy parameters. When the 512 kJ/m² index is used tocompare the fracture appearance for both sizes of specimens based on thenormalized energy parameter, it is apparent that 512 kJ/m² correspondsto approximately 4 percent shear fracture appearance for the standardspecimens, but to about 1 percent shear for the miniature specimens.

If total absorbed energy is compared in this way to fracture appearance,the disparity between the miniature and standard specimens is evengreater as shown in FIGS. 17 and 18. The miniature specimens at 41 Jcorrespond to more than 70 percent shear, compared to about 4 percentfor the standard specimens. The relative positions of the curves forstandard and miniature specimens are now interchanged. It can be seemfrom a comparison of FIGS. 17 and 18 that the miniature specimensrequire less total energy to achieve a given value of a percent shearthan the conventional CVNs. However, when the normalized energy is used,it is apparent that the miniature specimens seem to require more energyper unit fracture surface area to achieve a given level of percent shearthan do the conventional CVNs. This observation suggests that thereasonable results obtained from the area normalization could befortuitous, and may not hold for other materials.

These observations lead to the conclusion that using normalized energyas a harpy parameter and 512 kJ/m² as a Charpy index for miniaturespecimens is probably not generally applicable. The reasonable dataobtained may be attributed to the fortuitously small difference infracture appearance between standard and miniature specimens at the 512kJ/m² level, as is seen in FIGS. 15 and 16. Therefore, it is necessaryto establish a new Charpy parameter (other than fracture appearance) andan appropriate index that could be used to correlate miniature withlarge specimen energy vs temperature data.

                  TABLE IV.2                                                      ______________________________________                                        % SHEAR FRACTURE APPEARANCE                                                   COMPARED TO ENERGY (FROM FIGS. IV.1                                           THRU IV.4)                                                                                  % Shear F.A.                                                                           % Shear F.A                                                          @ 512 kJ/m.sup.2                                                                       @ 41 J                                                 ______________________________________                                        Standard Specimens.sup.1                                                                      4%         4%                                                 Miniature Specimens.sup.1                                                                     1%         70%                                                ______________________________________                                         .sup.1 All 3 Materials)                                                  

While fracture appearance can serve as a Charpy parameter for anyspecimen size, its measurement is tedious and often not consistent withthe requirements of current regulations. A new FMTB parameter and anappropriate index which makes use of load-deflection curves has beenfound. Besides being less tedious to measure than fracture appearance,the parameter obtained from the load-deflection traces is more amenableto automated or computerized data acquisition techniques.

It is well established that the total energy absorbed in fracturing aspecimen can be partitioned into pre-maximum and post-maximum loadenergies. The energy prior to maximum load can be portioned into:

(1) elastic stored energy

(2) crack formation energy, and

(3) plastic deformation energy.

The miniature specimen differs from the conventional one in size and ingeometry. Since the miniature specimen has a different span-to-widthratio, different anvil and punch geometry, and also has side grooves,the crack initiation energy and its ratio to total energy is differentin the two specimen types, and therefore not a useful index.

The post-maximum load energy can be partitioned into:

(1) elastic stored energy

(2) plastic deformation energy, and

(3) stable crack propagation energy.

The elastic energy is available to drive the cleavage fracture. Theremaining post-maximum load energy is associated with the plasticdeformation work and work that goes into propagating a stable crack.Therefore, the post-maximum load energy is less sensitive to differencesin specimen geometry and correlates well with fracture appearance, i.e.,percent shear. The onset of the maximum load corresponds approximatelyto crack initiation. This assumption is a reasonable one for somematerials and test temperatures. It is possible to measure crackformation using EP techniques.

By partitioning the total energy into premaximum and post-maximum loadenergies, and plotting these versus testing temperature, it can beshown, that the pre-maximum load energy does not show a conspicuoustransition in fracture behavior. As stated earlier, pre-maximum loadenergy is associated with elastic stored energy, crack initiation andplastic deformation near the notch. On the other hand, the post-maximumload energy exhibits a distinct transition. FIGS. 19 and 20 illustratethis behavior for the 600R-93 material.

Crack initiation can be defined as the complete formation of afull-width crack across the length of the notch defined here, iscomplete when maximum load is reached, the post-maximum load energyshould be proportional to percent shear for a given specimen size andshape regardless of material. Therefore the post-maximum load energy canbe used as a parameter for ductile fracture transition characterization.The differences in specimen size would result in the absolute values ofpost-maximum load energy being different between specimen types.However, the relative proportion of the energy that goes into crackpropagation and plastic deformation correlate with fracture appearancefor specimens of different sizes and shapes. It is necessary, therefore,to convert the absolute values of post-maximum load energy topercentages, by dividing by the total energy. The total energy used inthe calculations can be defined as the sum of the absorbed energy uptothe onset of cleavage or the energy absorbed in the formation of shearlips can be included or any other convenient energy measure. A plot ofpercent shear fracture appearance versus percent post-maximum loadenergy is given in FIG. 21. This figure shows that the data for all 3materials in both specimen geometries can be fit by the same curve withreasonable accuracy. This curve demonstrates that the new FMTBparameter, percent post-maximum energy, can correlate miniature andlarge specimen data. A correlation of this kind now allows for the useof two criteria, one based on energy, and the other on fractureappearance.

FIG. 21 provides the correlation for obtaining indices for both specimendimensions. The technique can be used to relate any specimen geometrieswhich yield fracture transition data.

Thus, from FIG. 17 for the standard CVNs, a Charpy energy level of 41Jourles corresponds to approximately 4% shear fracture appearance.Referring to FIG. 21, this level of shear corresponds to about 15%post-maximum load energy. Thus when fracture appearance is used as theCharpy parameter, the corresponding index is 4% shear. When percentpost-maximum load energy is chosen as the Charpy parameter, an index of15% post-maximum load energy is used as the index.

Therefore, when the parameters of percent shear fracture appearance orpercent post-maximum load energy are plotted versus test temperature(FIGS. 22 thru 27), transition temperatures for each material may beobtained at these new index levels (4% shear and 15% PME respectively).Table IV.3 summarizes these transition temperatures.

On comparing these transition temperatures with the standard CVN dynamictransition temperatures, the correction factors in the transitiontemperatures due to rate effect and size effect are obtained. Table IV.4shows these values. The average shift due to rate effect is 49.3 C (89.3F). The average shift due to size effect is 31.1 C (56.0 F).

FIG. 28 shows plots of energy versus test temperature for the threematerial heats and both specimen sizes. Table IV.5 gives approximatevalues of the Upper Shelf Energy (USE) in terms of Joules, kJ/m², andalso as the percent post-maximum load energy (100.0X.PME/TE).

                  TABLE IV.3                                                      ______________________________________                                        TRANSITION TEMPERATURES                                                       FROM % SHEAR AND FROM                                                          ##STR1##                                                                            60R-93    5A-91       60RR-5                                           Material Stand   Mini    Stand Mini  Stand Mini                               ______________________________________                                        15%      -15     -54     -57   -82   -65   -96                                 ##STR2##                                                                     6% SHEAR -18     -59     -62   -84   -73   -101                               ______________________________________                                    

                                      TABLE IV.4                                  __________________________________________________________________________    TRANSITION TEMPERATURE SHIFTS                                                 DUE TO RATE EFFECT AND SIZE EFFECT                                                                           (Size Eft)                                                             (Rate Effect)                                                                        Stnd Slw                                       Std Spec    Std Spec                                                                            Mni Spec                                                                            Stnd Spec                                                                            Bend to                                        Impct TT    Slw Bnd                                                                             Slw Bnd                                                                             Impct to Slw                                                                         Bend                                           [MANA85]    TT    TT    Bnd Shift                                                                            Shift                                          °C.  °C.                                                                          °C.                                                                          °C.                                                                           °C                                      __________________________________________________________________________    60R-93                                                                             40     -16   -57   56     41                                             5A-91                                                                              -7     -59   -53   44     26                                             60RR-5                                                                             -29    -69   -98   60     29                                             __________________________________________________________________________     Average Rate Effect Shift: 49.3 C (89.3F)0 C                                  Average Size Effect Shift: 31.1 C (56.0 F)                               

                  TABLE IV.5                                                      ______________________________________                                        UPPER SHELF ENERGY                                                            ______________________________________                                        UPPER SHELF                                                                   ENERGY [USE] FOR STANDARD SPECIMENS                                           Material Joules      kJ/m.sup.2                                                                            % PME/TE                                         ______________________________________                                        60R-93   80          1000    60%                                              5A-91    120         1500    69%                                              60RR-5   140         1750    69%                                              ______________________________________                                        UPPER SHELF                                                                   ENERGY [USE] FOR MINIATURE SPECIMENS                                          Material Joules      kJ/m.sup.2                                                                            % PME/TE                                         ______________________________________                                        60R-93   55          3690    72%                                              5A-91    45          3020    71%                                              60RR-5   40          2750    70%                                              ______________________________________                                    

FIG. 28 and Table IV.5 show that with respect to USE, the miniaturespecimen tests do not evaluate the 3 material heats in the same order asdo the conventional CVN tests. According to the conventional CVN tests,the Upper Shelf Energy shows increases from the most brittle material(60R-93) to the most ductile (60RR-5). This is the case whether the USEis measured in terms of Joules, kJ/m², or %PME/TE. However, in theminiature CVN tests, there is a decrease in the value of USE from 60R-93material to 60RR-5. As anticipated, %PME/TE does not show differencesbetween materials in the upper shelf region for either specimen size.

A general procedure for using miniature specimens to predict the shiftin Charpy transition temperature for each class of steel may besummarized as follows:

1. Perform dynamic or slow-bend tests to fracture on both the standardand the miniature specimens. Choose the temperature range of testing toensure data are obtained over the ductile-to-brittle transition.

2. Obtain for each specimen, as a function of test temperature, thefollowing data:

(a) Fracture Appearance, as the percentage of the total fracture surfacearea (excluding shear lips).

(b) Total Energy absorbed to cleavage fracture, in kJ/m2 or Joules

(c) The Post-Maximum Load Energy to cleavage fracture, in kJ/m2 orJoules.

(d) The Fractional Post-Maximum Load Energy, obtained by dividing thepost-maximum load energy by the total energy.

3. Plot Fracture Appearance versus Total Energy for the standardspecimens.

4. Obtain the value of Fracture Appearance (as a percent of shear)corresponding to the 41 J or (or 512 kJ/m2) standard use in the nuclearindustry.

5. Plot Fracture Appearance versus fractional Post-Maximum Load Energyfor all the specimens, both miniature and standard CVN. Fit the data toa single curve.

6. From the value of fracture Appearance obtained in 4, and the curveobtained in 5, get a value of Fractional Post-Maximum Energy. This valuethen corresponds to the 41 J standard.

(1) Brown, W. F., Jr. Lubahn, J. D., and Ebert, L. J., "Effects ofSection Size on the Static Notch Bar Tensile Properties of Mild SteelPlate", The Welding journal, Research Supplement, 554-s to 559-s(October 1947).

(2) Buffum, D. C., "Investigation of Square Sub-Sized V-Notched CharpySpecimens", ASTM Bulletin (TP 143), 45-47 (September 1949).

(3) Corwin, W. R., and Hoagland, A. M., "Effect of Specimen Size andMaterial Condition on the charpy Impact Properties of 9CR-1Mo-V-MbSteel", in The Use of Small Scale Specimens for Testing IrradiatedMaterial, ASTM STP No. 888, 325-338 (1986).

(4) Davidenkov, N., Shevandin, C., and Wittman, F., "The Influence ofSize on the Brittle Strength of Steel", Transactions ASME, Vol. 69, 63(1947)

(5) Grounes, M., "Review of Swedish Work on Irradiation Effects inPressure Vessel Steels and on Significance of Data Obtained", in Effectsof Radiation on Structural Metals, ASTM STP No. 426, 224-259 (1967).

(6) Jonassen, F., "Discussion on Effects of Section Size on the StaticNotch Bar Tensile Properties of Mild Steel Plate", The Welding Journal,Research Supplement, 27-s (January 1948).

(7) Lucas, G. E., Odette, G. R., Sheckherd, J. W., McConnell, P., andPerrin, J., "Subsized Bend and Charpy V-Notch Specimens for IrradiatedTesting", in The Use of Small Scale Specimens for Testing IrradiatedMaterial, ASTM (STP No. 888, 305-324 (1986).

(8) Macgregor, C. W., and Grossman, N., "Dimensional Effects inFracture", The Welding Journal, Research Supplement, 20-s to 26-s(January 1952).

(9) Orner, G. N., and Hartbower, C. E., "Effect of Specimen Geometry onCharpy Low-Blow Transition Temperature", The Welding Journal, ResearchSupplement, 521-s to 527-s (December 1957).

(10) Schwartzbart, H., and Sheehan, J. P., "Effect of specimen Size onNotched Bar Impact Properties of Quenched and Tempered Steels", ASTMProceedings, Vol. 54, 939-955 (1954).

(11) Wilsin, W. M., Hechtman, R. A., and Bruckner, W. H., "CleavageFracture of Ship Plates as Influenced by Size Effect", The WeldingJournal, Research Supplement, 200-s to 208-s (April 1948).

I claim:
 1. A process of determining the fracture mode transitionbehavior (FMTB) of solid materials, by loading the material,comprising:(a) providing a specimen with a side element, and having avolume and smallest dimension sufficient to establish continuum behaviorin all directions, and with a volume not more than 10⁷ times saidsufficient volume, said specimen having a notch and/or crack on the sideof the element; (b) modifying the stress field of the specimen byproviding at least one groove on the side element juxtaposed to thenotch and/or crack to provide overlapping stress field that includetransverse stress components which are approximately equal throughoutthe thickness of the side with the notch, when the specimen is loaded,resulting in measurable FMTB; (c) deforming the specimen by applying aload on the specimen by a punch moving at a velocity the same ordifferent from a preselected known standard to establish a correlationbetween percent shear and percent post-maximum energy for the specimenand the standard to define a post-maximum energy index between thespecimen and the standard which corresponds to the percent shearfracture of the standard, said load applied in the direction differentthan the orientation of the modified stress field; (d) measuring atleast one key variable in step c; and (e) determining the FMTB of thematerial from the measurements taken according to the principles of thefinite element method and/or the principles of linear or nonlinearmaterial mechanics.
 2. A process of determining the fracture modetransition behavior (FMTB) of solid materials, by loading the material,comprising:(a) providing a specimen with a side element, and having avolume and smallest dimension sufficient to establish continuum behaviorin all directions, and with a volume not more than 10⁷ times aidsufficient volume, said specimen having a notch and/or crack on the sideelement; (b) modifying the stress field of the specimen by providing atleast one groove on the side element juxtaposed to the notch and/orcrack to provide overlapping stress field that include transverse stresscomponents which are approximately equal throughout the thickness of theside with the notch, when the specimen is loaded, resulting inmeasurable FMTB; (c) deforming the specimen by applying a load on thespecimen in a direction different than the orientation of the modifiedstress field; (d) measuring at least one key variable in step c; (e)determining the FMTB of the material from the measurements takenaccording to the principles of the finite element method and/or theprinciples of linear or nonlinear material mechanics; (f) determiningthe specimen energy temperature data using either the fractureappearance or percent post-maximum energy parameters to define onadditive constant, (g) applying the miniature FMTB specimen data toaccount for rate effects and/or specimen size effects such that theminiature specimen data, with the additive correction data applied,yields conventional specimen data.
 3. A process of determining thefracture mode transition behavior (FMTB) of solid material, by loadingthe material, comprising:(a) providing a miniature specimen, having aplurality of sides and ends, of material having a volume and smallestdimension sufficient to establish continuum behavior in all directions,and with a volume not more than 10₇ times said sufficient volume, saidspecimen having a notch intermediate the ends on one side; (b) modifyingthe stress field of the specimen by providing grooves on two sidesopposite to the notch at a position intersecting the notch, so as toinduce stress in the specimen in a pre-selected field of orientation inthe notch; (c) deforming the specimen by applying a load of the specimenin a direction different than the orientation of the modified stressfield; (d) measuring at least one key variable in step c; (e)determining the FMTB of the material from the measurements takenaccording to the principles of the finite element method and/or theprinciples of linear or nonlinear material mechanics, (f) determiningthe specimen energy temperature data using either the fractureappearance or percent post-maximum energy parameters to define anadditive constant, (g) applying the miniature FMTB specimen data toaccount for rate effects and/or specimen size effects such that theminiature specimen data, with the additive correction data applied,yields conventional specimen data.